1. Field of the Invention
The present invention relates to a magnetic sticking sheet able to be supplied in a rolled state and a method of producing the sheet, more particularly relates to a magnetic sticking sheet suitable for printing by the sheet by feeding it from the rolled state and a method of producing the sheet.
2. Description of the Related Art
Magnetic sticking sheets using magnetic attraction of a magnet are widely used as various types of display tools. Particularly, they are widely used in offices.
In recent years, along with the rapid spread of personal computers, the performance of printers and other peripherals has been remarkably improved. The printing quality of personal printers is becoming comparable to the printing quality of business printers. In the field of business printers, demand for printers able to print on large size paper such as A0, A1, B0, B1 size paper has increased. At the same time, there is a growing desire to use such large size printed matter.
The most important use of large sized printed matter is posters. Posters are fixed to bulletin boards using various types of adhesives, adhesive tape, thumb tacks, capped magnets, and other fasteners. A magnetic sticking sheet poster is convenient in that the poster itself is a fastener having a magnetic sticking property. If the bulletin board has a ferromagnetic surface, no other fastener is needed. That is, the sheet can be fastened to the bulletin board on its own. Also, the sheet can be freely peeled off from the bulletin board.
Generally, magnetic sticking sheets are sheet type bond magnets. Along with their expanded applications, sheet type bond magnets have been made thinner. In recent years, magnetic sticking sheets produced by extrusion or injection molding having a thickness of the magnetic layer of about 0.1 mm and a total thickness of about 0.25 mm have been commercialized. These magnetic sticking sheets have axes of easy magnetization oriented perpendicularly to the surface of the magnetic layer and are magnetized perpendicularly. For example, U.S. Pat. No. 6,312,795 discloses a magnetic sheet of this type.
FIG. 1 shows schematically the magnetic layer 2 of the magnetic sticking sheet having an axis of easy magnetization perpendicular to the surface of the magnetic layer. As shown in FIG. 1, the magnetic layer 21 and attachment 9 are attached magnetically. The magnetic layer 21 is multipolar-magnetized at a certain pitch of magnetic poles. The N-poles and S-poles arranged alternately at an interface between the magnetic layer 21 and the attachment 9 generate a magnetic field shown by the magnetic lines of force 22.
A magnet generates a magnetic field outside it due to the N-poles and S-poles. On the other hand, the magnet also generates a magnetic field inside it due to the same magnetic poles. This is called a xe2x80x9cdemagnetizing fieldxe2x80x9d. The demagnetizing field faces the magnetic circuit formed by the outer magnetic field, so acts to demagnetize the magnet itself.
In the way that a magnetic field becomes stronger the shorter the distance between the N-S magnetic poles, the demagnetizing field becomes stronger and the magnet becomes more easily demagnetized the shorter the distance between the N-S magnetic poles.
As shown in FIG. 1, the conventional magnetic sticking sheet oriented and magnetized perpendicularly to the surface of the magnetic layer has a distance between magnetic poles equal to the thickness of the magnetic layer. Therefore, in order to increase the distance between magnetic poles and reduce the demagnetizing field, the thickness of the magnetic layer must be increased. On the other hand, when thinning the magnetic layer for the purpose of improving easiness of cutting and/or handling of the magnetic sticking sheet, the distance between magnetic poles consequently becomes short and demagnetizing field increases. Therefore, it becomes easy to be demagnetized.
Also, in the production of magnetic sticking sheets by extrusion, a paste containing a mixture of a particle type magnetic material and binder is processed at a high temperature and high pressure, so the equipment becomes large in size. In the case of injection molding, the thinner the magnetic sticking sheet, the more difficult it is to form and the greater the load on the equipment.
Further, since the conventional magnetic sticking sheet oriented and magnetized perpendicularly to the surface of the magnetic layer is so thick in total thickness as to be hard to roll and its magnetic sticking force is as high as 1.0 gf/cm2 or more, printing by printers for personal or business use is difficult. If printing on such magnetic sticking sheets by a printer for personal or business use in the same manner as printing on normal paper, the sheets would stick to each other making precise alignment and smooth feed impossible.
Particularly, when rolling magnetic sticking sheets having too strong a magnetic sticking force, the ends of the roll will become uneven or the roll will become slack. If magnetic sticking sheets are fed into a printer from a roll with uneven ends or having slackness, the magnetic sticking sheets will not be precisely positioned.
On the other hand, Japanese Patent No. 1460017 discloses a method of producing a magnetic sticking sheet including a step of coating a magnetic coating material containing magnetic particles to form a magnetic layer having a thickness of 0.1 to 0.3 mm, a step of orienting an axis of easy magnetization longitudinally (in-plane or parallel to a surface of the magnetic layer), and a step of multipolar-magnetization. It is described that the magnetic sticking force after magnetization is insufficient when the thickness of the magnetic layer is less than 0.1 mm. In practice, a sufficient magnetic sticking force is observed only at a 0.2 mm thickness of the magnetic layer in the embodiments of the patent. There is no description about the desirable squareness ratio in this patent. In this patent, a capacitor and yoke are used for magnetization.
In Japanese Unexamined Patent Publication (Kokai) No. 2001-76920 too, a flexible magnetic sheet having a magnetic film formed by coating a magnetic coating material containing hard magnetic particles is described. The flexible magnetic sheet has an axis of easy magnetization oriented longitudinally and is multipolar-magnetized longitudinally. In this publication, as an example of the method of multipolar-magnetization of the magnetic layer in a longitudinal direction, a method using a capacitor and yoke is mentioned.
This flexible magnet sheet can be made uniformly thin and be printed. As an example in the publication, a flexible magnetic sheet having a thickness of the magnetic layer of 0.07 mm and a sticking force of about 240 N/m2 (≈2.4 gf/cm2) is described.
The publication gives as examples including printing an example of printing a sheet cut to the A4 type size by a printer and an example of printing a sheet cut to a tape form by a thermal transfer type label writer. The publication does not describe a roll type sheet of a large size such as A0 applicable to high quality printing. Also, it does not investigate the characteristics of a magnetic sticking sheet suitable for feeding in a printer from a rolled state. When rolling a sheet having a magnetic sticking force equal to that of the above examples of the publication, their magnetic attraction force is too strong, the magnetic repulsive force has an effect, and shaping the roll becomes difficult. Therefore, it is impossible to print it normally by a printer.
When printing on paper having a size of for example A3 to A5, B4, B5, or so, a stack of paper cut in advance to the predetermined size is often used. In the case of an A0 type or other large size paper printer, however, if the paper is pre-cut and stacked, the area occupied by the printer will become remarkably large. Therefore, at present, roll paper is used for all of commercially available printers for large size paper.
As described above, the demand for large size paper printers has grown. A greater variety of paper is also demanded for such large size paper printers. To print on magnetic sticking sheets by a large size paper printer, the magnetic sticking sheets must be rolled. Therefore, it is necessary to make the magnetic sticking sheets as thin as normal paper and suppress the magnetic sticking force compared with a conventional magnetic sticking sheet. On the other hand, in consideration of use of a printed magnetic sticking sheet as a poster, the magnetic sticking sheet is required to have a magnetic sticking force able to support its own weight.
In addition to the above problems, the conventional method of producing a magnetic sticking sheet has another problem in that it consumes a large amount of electric power for magnetization and therefore is high in production cost. Magnetization of a magnetic sticking sheet requires a strong magnetic field. Up to now, as described in for example Japanese Patent No. 1460017 and Japanese Unexamined Patent Publication No. 2001-76920, magnetization has been performed by using a capacitor and yoke. The need for equipment for generating a strong magnetic field and the enormous amount of power consumed by the equipment remarkably increases the production cost of the magnetic sticking sheet.
Also, according to the methods of producing a flexible magnetic sheet described in Japanese Patent No. 1460017 and Japanese Unexamined Patent Publication No. 2001-76920, though a sheet having an axis of easy magnetization in a longitudinal direction to the magnetic layer is formed, a coated film with a magnetic coating material is dried after orienting the axis of easy magnetization. In other words, it is not dried in a magnetic field. In this case, it is difficult to raise the squareness ratio. This is disadvantageous for controlling the magnetic sticking force to within a desired range.
Summarizing the problems to be solved by the present invention, a conventional perpendicularly oriented and magnetized magnetic sticking sheet cannot be made thinner. Also, a conventional longitudinally oriented and magnetized magnetic sticking sheet is not suitable for rolling or feeding in printers from a rolled state. Further, the conventional method of producing a magnetic sticking sheet consumes too much electric power for magnetization.
An object of the present invention is to provide a magnetic sticking sheet having an axis of easy magnetization longitudinal to a magnetic layer reduced in demagnetizing field, resistant to demagnetization even when being made thin, resistant to poor rolling when rolled, and suitable for printing by a printer.
Another object of the present invention is to provide a method of producing a magnetic sticking sheet able to produce at low cost a rollable magnetic sticking sheet having a suitable magnetic sticking force.
According to a first aspect of the present invention, there is provided a magnetic sticking sheet comprising a non-magnetic base and a magnetic layer formed on the non-magnetic base by coating a magnetic coating material comprised of ferromagnetic particles dispersed in binder, a magnetic layer having a thickness of 0.03 to 0.10 mm, the magnetic layer having an axis of easy magnetization of the ferromagnetic particle oriented to give a squareness ratio of 80 to 90% in a parallel direction to a surface of the magnetic layer, the magnetic layer being multipolar-magnetized so that magnetization inverts alternately in a parallel direction to a surface of the magnetic layer, and the sheet having a total thickness of 0.08 to 0.25 mm including the thickness of the non-magnetic base, the sheet has enough flexibility to be rolled, a surface magnetic flux density of the magnetic layer of 35 to 100 Gauss (G), and a magnetic sticking force, required for removing a magnetic sticking sheet fixed magnetically on a magnetic surface via the magnetic layer while keeping the magnetic surface and the magnetic sticking sheet parallel, of 0.4 to 0.9 gf/cm2.
Accordingly, when rolling a long magnetic sticking sheet, the ends of the roll become uniform and the roll does not become slack.
According to a second aspect of the present invention, there is provided a method of producing a magnetic sticking sheet comprising the steps of coating on a non-magnetic base a magnetic coating material comprised of ferromagnetic particles dispersed in binder to form a coated film; orienting an axis of easy magnetization of the ferromagnetic particles in a parallel direction to the coated film by applying a magnetic field; drying the coated film while orienting the axis of easy magnetization by drying in the magnetic field to obtain a squareness ratio of 80 to 90% in the parallel direction to the coated film; further drying the coated film to form a magnetic layer; and multipolar-magnetizing the magnetic layer as the magnetization inverts alternately in the parallel direction to the magnetic layer, the step of multipolar-magnetization including the step of placing a combined permanent magnet comprised of a plurality of magnets stacked facing each other with different magnetic poles so as to face at least a side of the magnetic sticking sheet where the magnetic layer is formed.